Process liquid to be concentrated in such an evaporator plant may be e. g. juice from sugar beats, alternatively sugar cane, salt solutions, milk or fruit juice.

The Swedish patent document No. 196 988 describes with reference to Fig. 13 an evaporator plant comprising three pairs of plate heat exchang- ers, the plate heat exchangers of each pair being connected in parallel and the three pairs being connected in series.

The heat transfer plates in each plate heat exchanger in an evaporator plant of the relevant kind are provided with so called port holes, which form port channels through the plate package. The port channels commu- nicate with inlet and outlet openings in the plate heat exchanger. Further, the heat transfer plates are formed such that heat transfer passages are formed between the plates when they abut against each other in the plate package. Two separate sets of heat transfer passages are formed in the plate heat exchanger by means of every second heat transfer passage communicating with two first port channels and the remaining heat transfer passages communicating with two second port channels.

During operation the plate heat exchanger is flowed through by a heating fluid, usually water vapour, and a process liquid, which is to be concent- rated by means of evaporization of a part of the process liquid. The water vapour flows into the plate heat exchanger through an inlet opening, continuing through a port channel and through a first set of heat transfer passages. In the heat transfer passages the water vapour condenses at least partly to water in order finally to flow out of the plate heat exchanger via a port channel and an outlet opening. The process liquid flows into the plate heat exchanger through an inlet opening and a port channel and continues through the second set of heat transfer passages. In these heat transfer passages the process liquid is brought to boil by the water vapour in the adjacent heat transfer passages, concentrated process liquid and vapour being generated. Thereafter, the concentrated process liquid and the vapour flow out of the plate heat exchanger via a port channel and an outlet opening.

An evaporator plant of the described kind further comprises means, often stainless steel pipes, for conducting heating fluid and process liquid to and from the plate heat exchangers. A common conduit, alternatively a conduit from each plate heat exchanger, conducts concentrated process liquid and vapour from the plate heat exchangers to a separator, in which the concentrated process liquid is separated from the vapour. Conduits for concentrated process liquid and vapour have a relatively large diame- ter and, consequently, the cost for these conduits forms a significant part of the cost for the entire evaporator plant.

In an evaporator plant for concentrating a large amount of process liquid, a large number of heat transfer passages are required. Thus, if all these are to be included in one and the same plate heat exchanger, it must have quite a lot of heat transfer plates, which means that the heat

exchange fluids must pass through long port channels in the plate heat exchanger. In the port channel to be flowed through by concentrated process liquid and vapour, the pressure drop should be small, such that the amount of process liquid, which flows through the heat transfer passa- ges, will be essentially equal in different parts of the plate package. Often it is the through flow area of this port channel, which sets the upper limit for the length of the port channel and, thus, the limit for the number of heat transfer plates in the plate package. To achieve of a small pressure drop in the relevant port channel in a plate heat exchanger having many heat transfer plates, outlet openings for concentrated process liquid and vapour may be formed in both the frame plate and the pressure plate.

Accordingly, concentrated process liquid and vapour may flow through the port channel in two directions. By such an arrangement, the length of the port channel and, thus, the number of plates in the plate package, may essentially be doubled in comparison with there being only one outlet. However, a long conduit from the pressure plate of the plate heat exchanger to the separator and a short conduit from the frame plate of the plate heat exchanger to the separator, means that the process liquid is unevenly distributed between the heat transfer passages within the plate package due to different pressure drops in the conduits. Further, an evaporator plant formed in this way must comprise a long and cost increasing conduit having a relatively large diameter. Furthermore, conduits connected to the pressure plate of the plate heat exchanger result in more time required for dismantling of the plate heat exchanger, for instance in connection with cleaning thereof. Such cleaning is perfor- med regularly; in certain processes, for instance when concentrating juice from sugar cane, the interval between cleaning operations is as short as 10 days.

Alternatively, in an evaporator plant several plate heat exchangers may be used, each comprising a plate package with less heat transfer plates than the plate package in one single large plate heat exchanger.

However, also this results in a large number of often long conduits having a relatively large diameter. To connect several plate heat exchangers to one separator may also involve problems of proper arrangement of the plate heat exchangers and difficulties as to the access of the various parts of the evaporator plant for service and cleaning.

An object of the present invention is to accomplis an evaporator plant of the initially described kind, which avoids the above mentioned problem with several long conduits having a relatively large diameter. Also, an object with the invention is to make possible a space saving and service friendly arrangement of the various parts of the evaporator plant. These objects may be attained by means of an evaporator plant according to the invention, which is characterised in -that the said two plate heat exchangers are arranged with their respective outlet openings for concentrated process liquid and vapour essentially opposite to each other, and -that a common collecting pipe for both plate heat exchangers is arranged between the said outlet openings for concentrated process liquid and vapour of the plate heat exchangers, the collecting pipe being provided with a connecting device forming a fluid passage, which is adapted to communicate with a separator for freeing said vapour from particles of the said concentrated process liquid remaining therein and which has an area, which is larger than the area of the outlet openings for concentrated process liquid and vapour of each of the plate heat exchangers.

By the invention, only one conduit for connecting two plate heat exchangers to a separator is required in an evaporator plant of the present kind. The conduit may be of a desired length and is at one end thereof connected to the connecting device mentioned above and at the other end thereof connected to the separator. The two plate heat exchangers and the separator in an evaporator plant according to the invention are easily accessible for service.

Suitably, the said fluid passage has an area, which is essentially of the same size as the total area of the outlet openings for concentrated process liquid and vapour of both plate heat exchangers.

According to a preferred embodiment of the invention one of the collecting pipe and the connecting device comprises a connecting member, which forms a concentrate passage adapted for discharge of concentrated process liquid. The concentrate passage is suitably formed by a conduit, which may have a considerably smaller diameter than the conduit for concentrated process liquid and vapour, consequently, the cost for this further conduit is comparatively low. The two phase fluid has a larger flow resistance than a one phase fluid. By diverting a main part of the concentrated process liquid from the vapour as early as possible, the flow resistance in the respective conduits may be kept at a low level.

Suitably the said outlet openings for concentrated process liquid and vapour are formed in the respective frame plates of the plate heat exchangers. Each of the plate heat exchangers has an inlet opening for process liquid and an outlet and an inlet opening for a further fluid, which is intended to flow through the plate heat exchanger. Advantageously, all these openings are formed in the said frame plate of the plate heat exchanger. By forming the plate heat exchangers in this manner,

dismantling thereof is considerably facilitated. Namely, no conduits need to be dismantled at the pressure plates of the plate heat exchangers.

The frame plates of the two plate heat exchangers may be reversed regarding the location of the said outlet openings for concentrated process liquid and vapour. Thus, in an embodiment of the invention of this kind the frame plates are arranged opposite to each other, which makes a simple design of the collecting pipe possible. If the said outlet openings of the plate heat exchangers have a shape different than circular, they are suitably reversed regarding their shape, through which the forming of the collecting pipe is facilitated.

Depending upon the magnitude of the flow of a process liquid to be con- centrated in the evaporator plant, the plate heat exchangers may be either connected in parallel or connected in series regarding the intended flow of the said process liquid through the plate heat exchangers. At rela- tively small flows of process liquid, it is suitable to connect the plate heat exchangers in series, necessitating a smaller total amount of heat transfer plates in the two plate heat exchangers than if the plate heat exchangers would be connected in parallel. If the plate heat exchangers are connected in series, one of the said collecting pipe and the said connecting device may have a branching means for forming a channel adapted to be flowed through by partially concentrated process liquid and adapted to communicate with an inlet opening for process liquid in one of the plate heat exchangers. In the last mentioned embodiment of the invention, partially concentrated process liquid may in an easy way be conducted from one of the plate heat exchangers to the other while vapour from both plate heat exchangers may be discharged collectively.

The invention also concerns a collecting pipe of the above described kind.

The present invention is described closer below with reference to the accompanying drawings, in which -figs. 1-3 show previously known evaporator plants, -fig. 4 shows five heat transfer plates forming part of a plate heat exchanger in an evaporator plant according to the invention, -figs. 5 and 6 show two plate heat exchangers forming part of an evaporator plant according to the invention, -figs. 7 and 8 show a collecting pipe forming part of an evaporator plant according to the invention, and -fig. 9 shows a part of an evaporator plant according to the invention comprising a collecting pipe, which easily makes possible connection in series of two plate heat exchangers in the evaporator plant.

An evaporator plant for concentrating a process liquid may comprise a heat exchanger, a separator and conduits for conducting the process liquid and a heating fluid to and from the evaporator plant and between the various parts thereof. Advantageously, if the process liquid has a comparatively low content of fibres and other solid particles, a plate heat exchanger is used in the evaporator plant. Plate heat exchangers are compact and provide a maximum of heat transfer surface in relation to their space requirements. Additionally, plate heat exchangers are easily dismantled for cleaning or service. The separator, in which vapour is separated from concentrated process liquid, is usually a so called cyclone separator comprising a cylindrical separation chamber having preferably a vertically arranged centre axis. Concentrated process liquid and vapour are conducted tangentially into the separation chamber, the concentrated process liquid flowing nearest to the cylindrical delimiting surface of the separation chamber being collecte at the bottom of the separation chamber where it is discharged from of the separator. The vapour is

collecte in the middle of the separation chamber and is discharged from the separator through an outlet in the upper part of the separation chamber. Alternatively, the separator may be of another kind.

Figs. 1-3 show diagrammatically three previously known evaporator plants seen from above. For the sake of clarity, of the conduits only the outlet conduits for concentrated process liquid and vapour from the plate heat exchangers are shown.

Fig. 1 shows a plate heat exchanger 1 comprising a relatively long plate package 2 of heat transfer plates, which is clamped by means of so called tie bars (not shown) between a stationary frame plate 3 and a pressure plate 4 movable in relation thereto. The heat transfer plates and the pressure plate 4 are suspended from a supporting beam 5. In each of the frame plate 3 and the pressure plate 4 there is a common outlet for con- centrated process liquid and vapour. Opposite to these outlets in the plates 3 and 4, conduits 6 and 7 are arranged and adapted for discharging concentrated process liquid and vapour. At 8 the conduits 6 and 7 join to a common inlet conduit 9, which is connected with a sepa- rator 10 for separating vapour from concentrated process liquid.

The plate heat exchangers 1 in the evaporator plants shown in figs. 2 and 3 each comprises a plate package having a number of heat transfer plates about half of that of the plate package 2 of the plate heat exchanger in fig. 1. Therefore, in accordance with earlier discussions, one outlet for concentrated process liquid and vapour in each plate heat exchanger 1 is enough. In the evaporator plant according to fig. 2 each of the outlets is connected with an inlet conduit 9 to the separator 10 by means of a separate conduit. In fig. 3 each of the outlets of the plate heat exchangers 1 is connected with the separator 10 by means of a conduit.

Common to the evaporator plants in figs. 1-3 is that each comprises long, alternatively many, conduits for concentrated process liquid and vapour.

These conduits are often manufactured from stainless steel and have a diameter often exceeding 500 mm, which makes the conduits costly. The access to the various parts of the evaporator plant will suffer if the conduits are made shorter for decreasing manufacturing costs.

The heat transfer plates in a plate heat exchanger in an evaporator plant may be provided with four circular so called port holes, which form four port channels through the plate package. However, to make the most of the surface of the heat transfer plates it may be advisable to have port holes of a different shape and/or a different number of port holes. Fig. 4 shows an example of how heat transfer plates 11 from a part of a plate package in a plate heat exchanger forming part of an evaporator plant may be formed. The heat transfer plates 11 delimit by means of two kinds of gaskets 12,13 two sets of heat transfer passages 14,15 between the plates and five port channels 16-20 extending through the plate package.

Three of the port channels 16,17,18 communicate with the first set of heat transfer passages 14 and the two remaining port channels 19 and 20 communicate with the second set of heat transfer passages 15. The arrows in the respective port channels 16-20 indicate the flow direction of the fluids.

When the plate heat exchanger is in use in an evaporator plant water vapour constituting the heating fluid, flows through the port channel 16 and continues through the first set of heat transfer passages 14.

In the heat transfer passages 14 the water vapour condenses at least partially and flows out of the plate heat exchanger through the two port channels 17 and 18. Process liquid flows into the port channel 19 and continues through the second set of heat transfer passages 15, in which

the process liquid is brought to boil, concentrated process liquid and vapour being formed. Thereafter, the concentrated process liquid and vapour flow out of the plate heat exchanger via the port channel 20.

Instead of sealing against each other by means of gaskets 12, the heat transfer plates could be welded together in pairs. Then the gaskets 13 could seal between two such plate pairs. Only the heat transfer passages 15 for process liquid would in this case be accessible for cleaning. In such an arrangement the dismantling and above all the assembly of the plate package would be facilitated.

Fig. 5 shows from above two plate heat exchangers 21,22 forming part of an evaporator plant according to the present invention. The two plate heat exchangers 21,22 are arranged with their frame plates 23,24 opposite to each other. The frame plates 23,24 are reversed regarding the location of their inlet and outlet openings. An inlet 25 for water vapour is adapted in a conduit 26, which is common for both plate heat exchangers 21,22 and which communicates with an inlet opening for water vapour in each of the frame plates 23,24. Between the frame plates 23,24 a common collecting pipe 27 for concentrated process liquid and vapour is arranged.

The collecting pipe 27 is provided with a connecting device 28 forming a fluid passage adapted to communicate with a conduit for concentrated process liquid and vapour for conducting at least vapour to a separator.

The connecting device 28 is formed as a connecting piece provided with a flange.

Fig. 6 shows a side view of the evaporator plant in fig. 5. Between the frame plates 23,24 two outlet conduits for at least partially condensed water vapour are arranged, only one thereof 29 is visible in fig. 6. Additio- nally, an inlet conduit having an inlet 30 for process liquid is arranged between the frame plates 23,24. Besides being provided with the

connecting device 28 the collecting pipe 27 for concentrated process liquid and vapour is also provided with a connecting member 31 forming a concentrate passage adapted to communicate with a conduit for concent- rated process liquid. The connecting member 31 in this embodiment is in the form of a connecting piece. A conduit only for concentrated process liquid may have a relatively small diameter compared with the conduit for concentrated process liquid and vapour and, thus, is not particularly costly.

The connecting member 31 makes an early separation of a main part of the concentrated process liquid possible. All the concentrated process liquid is not conducted from the plate heat exchanger via the connecting member 31. Namely, a certain part of the concentrated process liquid accompanies the vapour in the form of droplets. The conduit, which is adapted to communicate with the concentrate passage in the connecting member 31 may conduct the concentrated process liquid to the separator, alternatively, it may conduct the concentrated process liquid directly to the next step in the process. The early separation entails that flow losses may be kept as low as possible in the conduit for concentrated process liquid and vapour between the collecting pipe 27 and the separator.

However, the connecting member 31 is not necessary for the invention. If the conduit for concentrated process liquid and vapour between the collecting pipe 27 and the separator is short and straight, all the concen- trated process liquid may be conducted through this conduit to the separator, alternatively, be separated immediately before the separator.

All the conduits between the frame plates 23,24 of the plate heat exchan- gers in figs. 5 and 6 are suitably provided with bellows or similar made from e. g. steel or rubber. These bellows are not necessary but facilitate

considerably the assembly of the plate heat exchangers 21,22 and the conduits 26,27,29, since the demand on the frame plates 23,24 to be parallel thereby does not need to be set as high.

Figs. 7 and 8 show the collecting pipe 27 in detail. The area of the ope- ning 32 of the connecting device 28 is essentially of the same size as the total area of the outlet openings for concentrated process liquid and vapour in the respective plate heat exchangers. The inlet openings 33,34 of the collecting pipe 27 are in this embodiment rectangular and, therefore, have a larger area than the outlet openings for concentrated process liquid and vapour in the respective plate heat exchanger, which outlet openings are assumed to have the same shape as the cross section of the port channels 20 in figure 4.

Depending on the process liquid to be concentrated and how much it is to be concentrated, the process liquid may either flow through both plate heat exchangers in parallel, as in figs. 5 and 6, or the process liquid may flow in series first through one heat exchanger and then through the other heat exchanger. Fig. 9 shows schematically how such a serial flow of process liquid through the plate heat exchangers may be obtained by means of a collecting pipe according to the invention. The process liquid is conducted into a first plate heat exchanger 35 via a conduit, not shown, in order to be partially concentrated, whereafter partially concentrated process liquid and steam flow into the collecting pipe 36. The partially concentrated process liquid flows via a branching means 37, forming a channel, and a conduit 38 to a second plate heat exchanger 39. If necessary, a pump 40 may be arranged in the conduit 38. The process liquid is concentrated to a final concentration in the second plate heat exchanger 39 and flows together with vapour out thereof into the collecting pipe 36. A main part of the finally concentrated process liquid is

conducted via the connecting member 41 to e. g. a separator. Vapour from both plate heat exchangers 35,39 and a mixture of droplets of concentrated process liquid are conducted via the connecting device 42 to the separator.

The invention is not limited to the above described embodiments. The collecting pipe 27; 36, the connecting device 28; 42, the connecting member 31; 41 and the branching means 37 may e. g. be formed as conduits having a larger extension than the shown connecting pieces.

The connecting device 28; 42, the connecting member 31; 41 and the branching means 37 may alternatively be constituted by openings in the collecting pipe 27; 36. The connecting member 31; 41 and the branching means 37 may be formed in the connection member 28; 42 instead of in the collecting pipe 27; 36. In the case where the frame plates 23,24 are not reversed regarding the location of the outlet openings for concen- trated process liquid and vapour, the frame plates 23,24 may be arranged opposite to each other and somewhat displaced laterally such that the collecting pipe 27; 36 may be formed in the same simple manner, as for instance according to the embodiment shown in figs. 7 and 8.